1. Field of the Invention
The present invention relates to an overvoltage protection system for the intermediate circuit of a frequency converter. In particular, the invention concerns an overvoltage protection system for a frequency converter provided with a small intermediate-circuit capacitor and controlled by pulse-width modulation (PWM).
2. Discussion of the Background
A PWM frequency converter provided with a small intermediate circuit capacitor is disclosed e.g. in specification U.S. Pat. No. B2-6,801,441. This specification describes a multi-phase voltage-controlled PWM frequency converter comprising a control unit, an uncontrolled mains bridge to be connected to a multi-phase alternating-current source, a direct-voltage intermediate circuit and a controlled load bridge for feeding a variable-frequency multi-phase alternating voltage to a multi-phase load, in which frequency converter the mains bridge is connected to the load bridge without a high-capacitance direct-voltage capacitor unit serving as an intermediate energy storage. The direct-voltage intermediate circuit of such a PWM-frequency converter is implemented using only a low-capacitance capacitor for limiting the voltage spikes caused in switching situations by the energy stored in leakage inductances.
The intermediate circuit of such a frequency converter must be provided with an overvoltage protection circuit in case of special situations, e.g. a failure of the mains supply.
An overvoltage protection solution for a frequency converter is described in specification DE-A1-102 57 096. This specification describes a frequency converter comprising a rectifier for rectifying a three-phase mains voltage to produce an intermediate-circuit direct voltage and an inverter for inverting the direct voltage of the direct-voltage circuit to produce a variable-frequency three-phase alternating voltage, which is used to feed e.g. an alternating-current motor. This specification proposes an overvoltage protection circuit to be connected in parallel with an intermediate-circuit capacitor having a relatively low capacitance value. The overvoltage protection circuit consists of a high-capacitance electrolytic capacitor and a switching device connected in series with it. The switching device comprises a series connection of a resistor and a diode and, connected in parallel with it, a switch consisting of resistors, a varistor and a thyristor.
In the application described in this DE specification, the electrolytic capacitor only has to withstand the momentary excess current spikes occurring in overvoltage situations, so it is considerably smaller in size and cost than a traditional capacitor connected directly to the intermediate circuit and rated on the basis of a full ripple and a sufficient service life. This solution works without any control circuits. The solution has the drawback of requiring additional components in the circuit, e.g. an electrolytic capacitor, which take up plenty of space and increase the costs of the device.
Another prior-art solution for intermediate-circuit overvoltage protection is to provide the intermediate circuit with a braking chopper in which a power semiconductor switch connects a resistor in parallel with the intermediate-circuit capacitor when its voltage exceeds an operating threshold. A drawback with this solution are the chopper components, which increase the costs of the device and take up plenty of space.
Other prior-art solutions for intermediate-circuit overvoltage protection are various overvoltage protectors, such as a so-called surge arrester, varistor or power Zener diode, connected directly in parallel with the intermediate-circuit capacitor:                A surge arrester-type protection device has the drawback that its voltage remains at a low value as long as a current is flowing, so it requires a series-connected power semiconductor switch to prevent it from receiving energy directly from the supply network (a solution of this type is described in patent application US2004246641).        Selecting the ratings of a varistor-type protection device may be difficult in some cases because it is a low-gradient device, in other words, the voltage difference between the leakage level and the actual clipping level is large.        A Zener-type protection device is of an ideal nature because its operating threshold is very sharp. However, a power Zener is a very expensive component.        